Energy transfer in an electronic identification system

ABSTRACT

A radio frequency transponder assembly comprises a passive transponder and an antenna with a matching circuit comprising an inductor and a capacitor which are formed integrally with the antenna components. The antenna is a dipole with legs formed from wire or foil strips, in which case the inductor and capacitor are formed from same material as the antenna legs. The values of the inductor and capacitor are calculated to match the antenna impedance to the equivalent load of the transponder circuit as seen via the power rectification circuits of the transponder.

BACKGROUND OF THE INVENTION

This invention relates to radio frequency transponders.

Passive radio frequency transponders derive energy from an interrogationsignal which is used to energise them, as opposed to active transponderswhich include an energy source such as a battery. This makes passivetransponders relatively cheap to manufacture. However, the lack of abuilt-in energy source and the need to extract energy from theinterrogation signal limits the effective operating range of suchtransponders for a given intensity of the interrogation energy field.Conversely, where significant operating ranges are called for, theamount of power which must be radiated by an antenna generating theenergy field increases, to the point where it may infringe certainsafety regulations or other legislation.

It is an object of the invention to provide a radio frequencytransponder arrangement which can operate at increased ranges or withlower power energising fields than prior transponders.

SUMMARY OF THE INVENTION

According to the invention there is provided a method of matching anelectronic circuit of a radio frequency transponder having a complexinput impedance to an antenna, the method comprising:

measuring the complex input impedance of the electronic circuit;

calculating a first matching element value to transform the complexinput impedance to an intermediate impedance value;

calculating a second matching element value to transform theintermediate impedance value to a real value corresponding to theantenna impedance; and

constructing a matching network with inductive and capacitive matchingelements corresponding to the calculated first and second matchingelement values.

The complex input impedance may have a negative reactance component,with the first matching element value being inductive and the secondmatching element value being capacitive.

The method may include constructing a matching network comprising acapacitor in parallel with the antenna and an inductor in series withthe antenna and the electronic circuit.

Preferably, the method includes forming at least one of the inductiveand capacitive elements of the matching network integrally with theantenna.

Further according to the invention a radio frequency transponderassembly comprises:

an antenna;

a transponder circuit including a power supply circuit arranged to befed with energy received by the antenna, a logic circuit, and amodulator circuit for generating data signals for transmission by theantenna; and

at least two impedance matching elements, at least one of the impedancematching elements being formed integrally with the antenna.

The transponder circuit may have a complex input impedance, with theimpedance matching elements comprising a capacitor and an inductor andtogether effectively defining an impedance matching circuit between theantenna and the transponder circuit.

Preferably, the complex input impedance of the transponder circuit has anegative reactance component and the antenna is a dipole or foldeddipole.

The matching circuit may comprise a capacitor connected in parallel withtheantenna and an inductor connected in series between the antenna andthe transponder circuit.

Preferably the capacitor is connected between first and second terminalsof the antenna in parallel with input terminals of the transpondercircuit.

The inductor is preferably connected in series with a first terminal ofthe antenna and an input terminal of the transponder circuit.

Typically, the antenna has at least one element comprising a firstlength ofconductive material, and the inductor comprises a second lengthof conductive material connected between the antenna and the transpondercircuit, the inductance thereof being determined by said second length.

For example, said at least one antenna element and said inductor maycomprise a conductive metallic foil.

In one version of the invention, the antenna has at least two elementsand the capacitor is formed by overlapping adjacent elements of theantenna separated by a dielectric layer.

Said at least one antenna element and said inductor may comprise ametallic wire.

Alternatively, the antenna may have at least two elements, with thecapacitor being formed from a length of wire connected to one element ofthe antenna and wound around another element thereof.

A preferred radio frequency transponder of the invention has anoperating frequency in the range of 440 to 930 MHz and an operatingrange of at least 1 m for an effective radiated power of a readerenergising the transponder of no more than 500 mW.

To produce low cost transponders, designers need a simple antenna systemto which is connected a transponder integrated circuit containing allthe electronic components. Such components would typical comprise therectifying diodes, storage capacitors, modulator, an internal lowfrequency oscillator, memory storage and logic circuitry. Typicaldesigns use voltage doubling rectifying circuitry to extract theoperating power, and backscatter modulation to relay the transponder'sdata to the reader This type of design results in only the rectifyingcircuitry and the modulator experiencing radio frequencyexcitation,while all other parts operate at a relatively low frequencygenerated by the oscillator.

Different countries regulate the use of the radio spectrum within theirzones according to their regional plans. These plans often limit thestrength of energising fields that may be used in their region, and thepresent invention is intended to allow transponders to operate at areasonable range from the source of the reader's energising field inthose regions where very low energising field powers are permitted.

In order to maximise the energy transfer from the energising field viathe antenna to the rectifying circuitry, ideally a matching network isneeded between the RF electronic components (rectifiers, modulators, andenergy storage), and the antenna. The matching network should convertthe actual complex input impedance of the transponder's electroniccomponents to the conjugate impedance of the antenna. In conventionaldesigns this might be achieved using transmission lines, or combinationsof inductors and capacitors.Due to the large values needed for theinductors and the capacitors and the poor manufacturing tolerancesachieved with integrated circuit manufacture, as well as the effects oftemperature on the nominal values of such components, it is notcommercially viable to include-these components inside theelectronicintegrated circuit.

Without a matching network a typical electronic circuit might exhibit aninput impedance of 8.86−j29.67 ohms while a dipole has an impedanceofapproximately 72−j0. At the junction of the antenna and the electroniccircuit the imbalance of the impedances would cause 70% of the incomingpower to be immediately reflected and not converted to useful energy bythe transponder. To compensate the energising field would need to beincreased by more than 3.3 times to deliver the same energy to therectifying diodes compared to that of a properly matched situation.

The present invention provides apparatus that provides the necessarymatching cost effectively, and a method that can be used to compute thecorrect values of the matching network.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic diagram showing the main components ofa conventional reader and passive transponder;

FIG. 2 shows the connection of a matching network of the inventionbetween an antenna and rectifying circuitry of the transponder;

FIG. 3 shows schematically a method of creating a matching network inorder to determine the complex impedance of the rectifying circuitry;

FIG. 4 shows test apparatus used to determine the complex inputimpedance of the rectifying circuitry;

FIG. 5 shows a Smith Chart illustrating the transformation of thecomplex impedance of the rectifying circuit by the matching networkcomponents to the output impedance of a signal generator;

FIG. 6 shows a matching network of the invention in schematic form;

FIG. 7 shows a Smith Chart illustrating the transformation of thecomplex impedance of the rectifying circuit of a transponder by thematching network components to the impedance of an antenna;

FIGS. 8a to 8 c show a first embodiment of a transponder assembly of theinvention with a dipole antenna having an integral matching network;

FIG. 9 shows a laboratory test set-up to measure the performance of thetransponder assembly;

FIG. 10 shows a second embodiment of a transponder assembly of theinvention;

FIG. 11 is a graph indicating the power available from a dipole antennaplaced in a 915 MHz energising field of 500 mW or 30 W at differentranges; and

FIG. 12 is a graph illustrating the output voltage with frequency of anantenna assembly with an integral matching network of the invention.

DESCRIPTION OF EMBODIMENTS

FIG. 1 shows the typical arrangement of circuitry for a transponder andinterrogator or reader system which relies on the transponder receivingpower from the energising field of the reader to power the transponder.This type of transponder is called “passive” as it contains no energysource and extracts power from the energising field to operate.

Referring to FIG. 1, a radio frequency signal from an oscillator 10operating at the designated transponder operating frequency or carrierfrequency is boosted in a power amplifier 12 and radiated via atransmitter antenna 14. This creates an energising field 8. Thetransponder consists of an antenna 16 and an electronic circuit 36attached to the antenna. The electronic circuit includes all thefunctional components of the transponder, such as a power extractioncircuit or power rectifier 18 for deriving power from the energisingfield, a modulator 20 for sending data from the transponder back to thereader, and a logic circuit 22 including a low frequency internaloscillator, typically operating at 10 kHz, memory, and a control logiccircuit.

The present invention aims to enhance the conversion of energy collectedfrom the energising field to provide the necessary operating voltageneeded for the operation of the transponder, as this parameter has thelargest impact on the operating range achievable with a transponder fora given energising field power.

Using well known radio equations, the power at the output of thetransponder antenna is$P_{Transponder} = \frac{{Power}_{Amplifier}*{Transmitter}\quad {antenna}\quad {gain}*{Transponder}\quad {aperture}}{4*\pi*{Range}^{2}}$

This power is delivered to the transponder at the impedance of theantenna which depends on the properties of the antenna. For example, adipole has an impedance of 72+j0 ohms, while a folded dipole has animpedance of 300+j0 ohms.

The RMS voltage available at the ports of the antenna is${Voltage}_{Available} = \sqrt{{Power}_{Transponder}*{Impedance}_{Antenna}}$

The equations above apply to the situation where all the available poweris converted to voltage and not lost through impedance mismatch.

From the above equations the relationship between delivered voltage andrange is ${Voltage}_{unavailable}\quad \alpha \quad \frac{1}{Range}$

The actual operating current of the electronic circuitry in atransponder of the kind in question is usually very small, for example10 μA at 1.5 volts, giving an equivalent load resistance of 150 kΩ. Byproviding a practical impedance matching network to match thatimpedance, as seen through the rectifying diodes of the power rectifier,to the transponder antenna impedance, it is possible to deliver muchhigher operating voltages and thereby to increase the range at which thetransponder can operate for a given energising field.

FIG. 2 shows a typical power rectification circuit that might be used ina transponder to supply sufficient voltage and current to the equivalentload 24 of the transponder. A suitable matching circuit or network 26converts the antenna impedance to the conjugate impedance of theequivalent load as transformed by the impedances of the rectifyingdiodes D1 and D2 and an energy storage capacitor C.

It is possible with sophisticated network analyzers and extensivemathematical equations to compute suitable values for the matchingnetwork, However a much simpler practical method of determining thesevalues is possible.

Radio frequency engineers have been using for many years the graphicalcomputation charts called “Smith Charts” (trade mark) invented by PhilipSmith prior to 1932 and sold by Analog Instruments. The use of thesecharts will be known to radio engineers who are taught its operation aspart of their training.

The Smith chart graphically shows how impedances are converted from somecomplex source impedance to a complex load impedance. The chart worksfor all impedances. Generally a reference impedance is chosen and allvalues plotted on the chart are normalised to this impedance, with allpoints being plotted as ratios.

Due to the characteristics of the rectifying circuit 18 comprising thediodes D1 and D2 and the capacitor C, which are dominant compared to theequivalent load 24, and the relatively high impedance of the modulator20 (typically 10 kΩ when “on” and 1 MΩ when “off”), the impedance of therectifying circuit and hence the effective input impedance of thetransponder circuit as seen from the matching network 26 will lie in thelower half of the Smith Chart in the “Negative Reactance sector” (SeeFIG. 5). In other words, the input impedance will be complex and willcomprise a positive real component with a negative imaginary component.Matching occurs by moving this impedance to the “Positive Reactancesector” by means of a series transmission line or an inductor, and thentranslating it to the source impedance by means of a capacitor. Thisprocess requires two precise elements, the first to move the impedanceto a transition point or intermediate impedance value in the correctpart of the sector, and the second to move it from the transition pointto the load.

FIG. 3 shows the conceptual circuit needed, comprising a parallelcapacitor C, and a transmission line 28. There are other ways ofmatching, but the prime purpose of the invention is to provide apractical way of matching the transponder chip to the antenna and thematching network should preferably therefore be included in theconstruction of the antenna, preferably using the same materials as usedin the antenna, so as to minimise production costs. As the Smith Chartallows the use of a transmission line or an inductor to translate fromthe Negative to the Positive sector, test measurements can be madeaccurately using a transmission line and then implemented practicallyusing an inductor. For the purpose of determining the actual inputimpedance of the transponder circuit in a measurement laboratory, testequipment can be used operating at a different impedance from theeventual antenna circuitry, and once the correct input impedance of thetransponder circuit has been determined, a new matching circuit can beimplemented to match to any chosen antenna impedance.

FIG. 4 shows a test set-up for implementing the arrangement of FIG. 3,wherein a signal generator 30 operating at the correct frequency isconnected to a jig comprising a 50 ohm transmission line 28 and avariable capacitor C_(L) which is connected to the integrated circuit 36to be matched. Ideally a voltmeter 34 is used to measure the voltageapplied to the equivalent load 24. In systems where a voltage output isnot available, the functioning of the circuit is monitored and the powerfrom the signal generator is reduced until the circuit just stopsworking. In this manner the quantitative effectiveness of the matchingcircuit can be assessed.

By using test matching circuits with different transmission line lengthsand in each case tuning the variable capacitor to minimise the signalgenerator power needed to operate the circuit or supply the same voltageon the equivalent load, it is possible empirically to determine thecorrect transmission line length and the value required from the tuningcapacitor for maximum energy transfer. This maximum will occur when thematching network matches the impedance of the rectifying circuit to thesource impedance of the generator. This matching process is shown in theSmith Chart diagram of FIG. 5. From the value of the tuning capacitorand the length of the optimum transmission line, the complex inputimpedance of the transponder circuit at the operating voltage can bedetermined.

An antenna system that will be simple to manufacture can be chosen. Thismight typically be a dipole with an impedance of 72+0jΩ. Using the SmithChart and now knowing the impedance of the rectifying circuit, a newmatching circuit can be designed based on a parallel capacitor C and aseries inductor Las shown in FIG. 6. The values of these components arecalculated according to the Smith Chart calculation ox FIG. 7. By makinga small change to the inductor and the capacitor values, almost anyantenna design can be matched with this network. For example, theantenna could be a shortened dipole, or another electrically “short”antenna, with the abovementioned matching process readily accommodatingsuch antennas' complex impedance.

FIG. 8 shows a practical embodiment of an antenna/transponder assemblyaccording to the invention. FIG. 8a shows the components of theassembly, which is based on an integrated circuit transponder chip 36and a dipole antenna having a nominal 915 MHz operating frequency.

The dipole antenna consists of first and second limbs 38 and 40 formedfrom a thin metallic foil strip 5 mm wide. In the prototype, a tinnedcopper foil was used for ease of soldering, but other metal foils ormetallic inks or other coatings on a suitable substrate could be usedinstead. The limb 38 is 80 mm long, while the limb 40 is 100 mm long,but the strips overlap by 20 mm when assembled so that each leg of theantenna is effectively 80 mm in length. Attached to the innermost end 42of the leg 38 is an inductor comprising a loop 44 formed from 0.5 mmwide foil tape. The loop is folded back on itself and the total lengththereof is 21 mm. The loop has a free end 46 which is connected to aninput terminal of the transponder chip 36, as described below.

A further component of the assembly is a strip of dielectric tape 48which is 20 mm long and at least 5 mm wide.

As shown in FIGS. 8b and 8 c, the dielectric tape 48 is attached to theinner end portion 42 of the leg 38 of the dipole, and the correspondingend portion 50 of the leg 40 is placed in position on top of it. Theends 42 and 50 of the respective legs 38 and 40 do not come intoelectrical contact with one another,and a capacitor is therefore definedbetween them with a value determined by the area of the overlappingportions of the foil strips and the thickness of the dielectric tape 48and its dielectric constant. The free end 46 of the loop 44 is solderedor otherwise connected to an input terminal 52 of the integrated circuittransponder chip 36, while a second input terminal 54 of the transponderis connected to the leg 40 of the dipole, close to the zone of overlapwith theleg 38. The whole assembly is preferably secured to a substratein the form of a strip 56 of paper or thin plastic sheet material, forexample, to make the assembly mechanically robust. A protective layer 58comprising a plastics or epoxy-based adhesive can be applied over thecentral portion of the assembly in order to improve its strength andweather resistance.

This arrangement effectively connects the transponder integrated circuitto a dipole antenna with an inductor in series between one leg of thedipole and thetransponder input terminal and a capacitor connectedacross the two legs of the dipole. The circuit is shown schematically inFIG. 6.

The transponder in question has a nominal operating voltage of 1.5 voltsand draws approximately 10 μA. The effective input impedance of thetransponder chip at its input terminals 52 and 54 was measured at8.86−j29.67Ω ahead of the internal rectifying circuit of thetransponder. At 915 MHz, an inductor with a value of approximately 9 nHand a capacitor with a value of approximately 6 pF were required toprovide the necessary impedance matching to the 72 ohm dipole, and theabove described physical dimensions of the antenna/transponder assemblywere calculated to give these values.

FIG. 9 shows a set-up for testing the efficiency of the matching networkwhich allows fine adjustments to optimise the matching network. Thetransponder assembly is set up at a fixed distance from the transmitantenna 14 of the reader and the output of the power amplifier 12 variedto achieve theoperating conditions of the transponder, ie. an operatingvoltage of at least 1.5 V. The matching circuit can be fine tuned andthe power reduced as optimal conditions are met.

FIG. 10 shows an alternative embodiment of an antenna/transponderassembly according to the invention. In this embodiment, fine enamelledwire of 0.4 mm diameter is used instead of foil strips to form theantenna and the impedance matching components. Again, a dipole antennahaving 80 mm long legs 60 and 62 is provided. The inner end 64 of theleg 60 terminates in a loop 66 having an effective length of 21 mm and afree end 68 which connected to the terminal 52 of the transponder chip36.

This loop defines an inductor in series with the leg 60 and the inputterminal 52 of the transponder chip. The other leg 62 is connected at apoint 70 to the input terminal 54 of the transponder chip and isextended past the terminal 52 and wound around the inner end 64 of theleg 60 in a coil 72 having ten turns, to form a parallel capacitor. Thecentral portion of the assembly is covered with a protective coating 74of plastics or epoxy resin, for example. The performance of thisembodiment is substantially similar to that shown in FIGS. 8a to c.

The described capacitive/inductive matching network provided aneffective impedance for the power to voltage conversion equation$\left( {P = \frac{V^{2}}{Z}} \right)$

of about 4,500 ohms. Using a signal generator with a 50 ohm outputimpedance, tests showed that under conditions where 11.2 mW (+10.5 dBm)was required to obtain an operating voltage of 1.5 volts after thetransponder rectifier circuit, only 126 μW (−9 dBm) was required whenthe above described matching circuit was used.

Comparing the operation of a transponder assembly using a standarddipole at a fixed distance from the transmit antenna 14, with andwithout the matching circuit, the assembly utilising the matchingcircuit required 19 dB less radiated power for equivalent operation. Asindicated in FIG. 11, which shows the power available from a dipoleantenna feeding a transponder of the kind in question, this allows anenergising field which is within the European requirements of 500 mWeffective radiated power (ERP) still to have an effective operatingrange of 1.6 m at 915 MHz. As shown in FIG. 12 the bandwidth of thematching circuit is almost 100 MHz for a system designed to operate at860-930 MHz, meaning that the same circuit can be used successfully indifferent markets where different interrogator frequencies areprescribed.

Although the invention has been described with reference to the use of adipole antenna, the invention also lends itself to the use of foldeddipoles or electrically short antennas.

What is claimed is:
 1. A method of matching an electronic circuit of aradio frequency transponder having a complex input impedance to anantenna, the method comprising: measuring the complex input impedance ofthe electronic circuit; calculating a first matching element value totransform the complex input impedance to an intermediate impedancevalue; calculating a second matching element value to transform theintermediate impedance value to a real value corresponding to theantenna impedance; and constructing a matching network with inductiveand capacitive matching elements corresponding to the calculated firstand second matching element values, including forming at least one ofthe inductive and capacitive elements of the matching network integrallywith the antenna.
 2. A method according to claim 1, wherein the complexinput impedance has a negative reactance component, and wherein thefirst matching element value is inductive and the second matchingelement value is capacitive.
 3. A method according to claim 2, includingconstructing a matching network comprising a capacitor in parallel withthe antenna and an inductor in series with the antenna and theelectronic circuit.
 4. A passive radio frequency transponder assembly ofthe kind that extracts energy for its operation from an energizingfield, the transponder assembly comprising an antenna; a transpondercircuit, the transponder circuit including a power supply circuitarranged to be fed with energy received by the antenna, a logic circuit,and a modulator circuit for generating data signals for transmission bythe antenna; and an impedance matching circuit effectively disposedbetween the antenna and the transponder circuit, wherein the transponderassembly is secured to a substrate, the antenna comprises at least onelimb of conductive material supported by the substrate, the impedancematching network includes an inductive component formed from the samematerial as that of the antenna and attached to a limb of the antenna,and the matching circuit is optimized to enhance the transfer of energycollected by the antenna to the power supply circuit of the transpondercircuit.
 5. A radio frequency transponder assembly according to claim 4,wherein the transponder circuit has a complex input impedance, andwherein the impedance matching elements comprise a capacitor and aninductor.
 6. A radio frequency transponder assembly according to claim5, wherein the complex input impedance of the transponder circuit has anegative reactance component and the antenna is a dipole or foldeddipole.
 7. A radio frequency transponder assembly according to claim 6,wherein the matching circuit effectively comprises a capacitor connectedin parallel with the antenna and an inductor connected in series betweenthe antenna and the transponder circuit.
 8. A radio frequencytransponder assembly according to claim 7, wherein the capacitor iseffectively connected between first and second terminals of the antennain parallel with input terminals of the transponder circuit.
 9. A radiofrequency transponder assembly according to claim 8, wherein theinductor is effectively connected in series with a first terminal of theantenna and an input terminal of the transponder circuit.
 10. A radiofrequency transponder assembly according to claim 9, wherein the antennahas at least one element comprising a first length of conductivematerial, and the inductor comprises a second length of conductivematerial connected between the antenna and the transponder circuit, theinductance thereof being determined by said second length.
 11. A radiofrequency transponder assembly according to claim 10, wherein said atleast one antenna element and said inductor comprise a conductivemetallic foil.
 12. A radio frequency transponder assembly according toclaim 11, wherein the antenna has at least two elements and thecapacitor is formed by overlapping adjacent elements of the antennaseparated by a dielectric layer.
 13. A radio frequency transponderassembly according to claim 10, wherein said at least one antennaelement and said inductor comprises a metallic wire.
 14. A radiofrequency transponder assembly according to claim 13, wherein theantenna has at least two elements and the capacitor is formed from alength of wire connected to one element of the antenna and wound aroundanother element thereof.
 15. A radio frequency transponder assemblyaccording to claim 4, having an operating frequency in the range of 440to 930 MHZ and an operating range of at least 1 m for an effectiveradiated power of a reader energizing the transponder of no more than500 mW.